The present invention relates to an ignition timing control apparatus of an internal combustion engine of the spark ignition type.
When the ignition timing of an engine is to be controlled based upon a spark advancing value which is digitally calculated relying upon various signals which indicate the operating condition of the engine, for example, relying upon rotational speed signals which indicate the rotational speed of the engine, intake amount signals which indicate the amount of the air sucked into the engine, and the like, it is accepted practice either to calculate, from the spark advancing value, a crank angle between a reference angle of the crankshaft and a crank angle corresponding to the ignition timing, or to calculate a value which corresponds to the period of time required by the crankshaft to rotate from a reference crank angle to a crank angle corresponding to the ignition timing. The value obtained by either calculation is preset, for example, in a downcounter and is counted down in response to clock signals which are produced by crank rotation of a predetermined angular increment or after every time interval of a predetermined length, respectively. The operation of the count down is started from the moment at which the crankshaft reaches the above-mentioned reference angle position, and the moment at which the counted value of the downcounter becomes zero determines an ignition spark timing.
However, when the engine is being cranked at starting, since the rotational speed, the intake vacuum and the intake air amount vary to a large extent, the ignition timing calculated in response to these various operating conditions changes greatly. Therefore, according to the above-mentioned conventional ignition timing control apparatus, stable engine operation cannot be obtained during starting.